An active matrix substrate used for a liquid crystal display device or the like includes a switching element such as a thin film transistor (hereinafter, referred to as a “TFT”) for each of pixels. Conventionally, a TFT including an active layer formed of an amorphous silicon film (hereinafter, such a TFT will be referred to as an “amorphous silicon TFT”) or a TFT including an active layer formed of a polycrystalline silicon film (hereinafter, such a TFT will be referred to as a “polycrystalline silicon TFT”) is widely used as such a switching element.
Recently, there has been an attempt to use a material other than amorphous silicon or polycrystalline silicon as a material of an active layer of a TFT. For example, Patent Document 1 describes a liquid crystal display device in which an active layer of a TFT is formed of an oxide semiconductor film of InGaZnO (oxide containing indium, gallium and zinc) or the like. Such a TFT is referred to as an “oxide semiconductor TFT”.
The oxide semiconductor TFT is operable at a higher speed than an amorphous silicon TFT. The oxide semiconductor film is formed by a simpler process than a polycrystalline silicon film, and therefore, is applicable for a device which needs to have a large area size. The oxide semiconductor TFT is now being progressively used for a display device as a high performance active element that is produced with a smaller number of production steps and a lower production cost.
An oxide semiconductor has a high mobility. Therefore, the oxide semiconductor TFT provides performance of a level higher than, or equal to, that of a conventional amorphous silicon TFT even if having a smaller size. For this reason, in the case where the oxide semiconductor TFT is used, the ratio of area size of the TFT with respect to a pixel is decreased while the numerical aperture of the pixel is improved. This allows bright display to be provided even if the light amount of backlight is suppressed, and thus lowers power consumption.
For a compact and high-definition display device used for, for example, a smartphone or the like, it is not easy to improve the numerical aperture of the pixels because of the restriction on the minimum width of the lines (process rule) or the like. In such a situation, an oxide semiconductor TFT may be used to improve the numerical aperture of the pixels, so that high definition image display is provided with low power consumption even in a compact display device.
An oxide semiconductor TFT has a superb off-leak characteristic, and thus the pixel voltage is generally kept at a constant level even during a period in which the TFT is off. This allows use of an operation mode of decreasing the rewrite frequency for providing display. For example, the display device is made operable so as to rewrite image data at a frequency of once per second (1 Hz) for displaying a still image. Such a driving system, which is called an “off driving system” or a “low frequency driving system”, significantly decreases the power consumption of the display device.